We have found a number of novel cellular processes that occur during the sexual reproductive cycle of an effective model system, Chlamydomonas. These include (1) the striking alterations of the macromolecular composition of nuclear DNA synthesized during gametogenesis, zygote maturation and gametic de-differentiation, (2) the preferential conservation, at the completion of the sexual reproductive cycle, of vegetative cell nuclear DNA molecules over those synthesized during gametogenesis and zygote maturation, (3) the restriction and subsequent degradation of the redundant gametic organelle DNA molecules during zygotic development, (4) the inhibiting effect of 5 bromo-deoxyuridine on the methylation of certain, but not all, species of DNA synthesized during zygote development, (5) the de novo synthesis and subsequent rapid turnover of gametic ribosomes in the midst of extensive degradation of pre-existing vegetative cell ribosomes during gametogenesis, (6) the complex variation in the cellular expressions of a DNA biosynthesis enzyme activity during the sexual reproductive cycle, and (7) the sequential expression of pre-programmed developmental potentials similar to that observed during normal zygote development in the de-differentiation of un-mated but differentially aged gametes. We plan to conduct, initially, in depth basic studies at the molecular and cellular level using biochemical, biophysical and genetic methods to characterize and to elucidate the mechanisms of these and other cellular processes that are not usually observed during vegetative growth. The results obtained from these studies will be applied immediately to probe the functional and causal, interrelationships between those novel processes that we have observed and the initiation and regulation of the sexual reproductive cycle. Subsequent to these basic studies we plan to investigate whether the induction of gametogenesis and meiosis, the fusion of gametes and zygote development, the frequency of synaptic crossing-over of chromosomal DNA and the physical inheritance pattern of organelle DNA.